The present invention relates to an optical data storage disc.
Optical data storage discs, particularly compact discs (CD's or CD-ROM's), comprise a disc of transparent material formed over one side with a pattern of indentations (or "pits") defining digital data, and this indented side of the disc is provided with a thin coating of reflective material. The digital data is optically "read" from the opposite side of the disc.
In order to increase the density of the data stored on the disc, the indentations must be smaller and more closely packed. It has now been agreed by a major consortium that the thickness of the disc should be reduced in thickness from the usual 1.2 mm, to approximately 0.6 mm: this agreement is with a view to forming discs of very high data density, which require the reduced thickness in order that the indentations can be resolved and optically read. However, the strength and rigidity of the reduced-thickness disc then needs to be restored by adhering a reinforcing disc over its indented side.
We have previously developed techniques for applying holograms to conventional CD's and CD-ROM's, not only as a decoration but more importantly as a protection against counterfeiting. We have now devised techniques for providing high density optical data storage discs (e.g. DVD's--digital versatile discs) with holograms or other optically variable images.